WASHINGTON (CNN) --As thousands of Iraqis converged on Najaf on Sunday to mourn the bombing death of a Shiite cleric, two U.S. senators accused the Bush administration of going to war in Iraq without a plan for peace.

At least 83 people were killed in Friday's bombing outside Imam Ali Mosque, considered sacred by Shiites, Najaf's governor said Sunday. Among the dead was Ayatollah Mohammad Baqir al-Hakim, spiritual leader of the Supreme Council of the Islamic Revolution in Iraq. (Full story)

The attack in Najaf was the latest in a spate of violence in the country, including attacks in Baghdad on U.N. headquarters and the Jordanian Embassy.

"We need someone who understands that you don't rush into war without a plan for peace and who knows how to make that plan effective," Massachusetts Sen. John Kerry, a Democrat, said on NBC's "Meet the Press."

Kerry, expected to formally announce his presidential candidacy Tuesday, said U.S. troops should remain in Iraq, but he does not believe more should be dispatched.

Siding with Kerry was another Democratic candidate, Sen. Joseph Lieberman of Connecticut.

Lieberman said on CBS's "Face the Nation" that the overthrow of Saddam Hussein was justified, but the Bush administration failed to plan beyond the war.

"Now we're faced with a real crisis in Iraq," Lieberman said. "We're in danger, the Bush administration having won the war, of losing the peace in a way that will be a terrible setback in the war against terrorism and will endanger the lives of more American soldiers over there."

Lieberman expressed disappointment that Bush had not gotten commitments from other nations to join in the postwar effort, even if they did not join the battle.

"I am shocked at how unprepared the Bush administration was for what to do afterward," Lieberman said. "They've left a vacuum which the terrorists, the Saddam loyalists, our enemies, have jumped into."

He and Kerry said Congress should spend whatever it takes to rebuild Iraq and carry out commitments that the United States made to the Iraqi people.

In response to a question, Lieberman said the United States should take France up on its recent offer to aid peacekeeping in Iraq, if only to show other nations that the United States has no intention of controlling the country.

But France's offer was conditional on the United States letting the U.N. Security Council manage the country, a scenario the Bush administration has said it opposes.

Lieberman said more troops are needed in Iraq, though not necessarily U.S. forces.

He added that the United States has to convince the United Nations and Muslims "that we're all on the same side. ... Don't think that the terrorists just hate America," he said. "They hate the whole civilized world."

Speaking on "Fox News Sunday," Republican Sen. Richard Lugar of Indiana said he was concerned about the death of al-Hakim, calling him "a bridge" among factions in Iraq, including Americans, who must be replaced quickly.

The chairman of the Foreign Relations Committee said al-Hakim represented a stabilizing force among the Shiites, Iraq's religious majority.

"Whoever did that killing knew [that bridge]," he said.

Lugar urged Bush to prepare a five-year plan for Iraq and let Congress and the public know how much money will be needed to rebuild the country. Other nations should be urged to help, he said.